The present invention relates to coherent correlation radar systems and more particularly to coherent correlation radar systems wherein the detection and correlation of the second order doppler is utilized.
In prior known practices the three most prevalent forms of coherent pulse systems are the homodyne delayed-local oscillator (DLO), the superheterodyne DLO, and the ZERO fuze systems. The homodyne DLO fuze processes the target-returned signal which is present about the fundamental harmonic at the output of the RF mixer. This signal is in the 1/f noise spectrum of the mixer crystals which limits the ultimate sensitivity of the system. In addition, the missile flight environment is capable of producing microphonics in this same frequency range presenting a possible prefunctioning condition. These shortcomings can be avoided by using superheterodyne techniques. However, all DLO systems have one characteristic that frequently poses a serious problem; viz., the need to delay internally a sample of the transmitted RF signal for an interval equivalent to the maximum range at which target detection is required. This delay is usually obtained with a length of coaxial cable, the volume and transmission loss of which may be excessive for any but the shortest range gates, especially at higher microwave frequencies.
The ZERO fuze system requires no RF delay line in its implementation, since target-return signals are sampled in temporal coincidence with the transmit pulse. However, for this reason, the ultimate sensitivity of the system is limited by the transmitted energy coupled into the receiver antenna system via the missile skin. An additional problem encountered in implementing th practical ZERO fuze is that of obtaining and maintaining the required stability.
The maximum fuzing range which can be obtained in both DLO and ZERO systems in which doppler processing is employed is limited by two conflicting requirements: (1) Since the maximum target doppler frequency must be sampled at least twice per cycle, there is a lower limit to permissible modulation rep-rate; and (2) Since the maximum range is inversely proportional to modulation rep-rate, there is also an effective upper limit.
The above described systems are also limited in range of operation by the sampling rate required by the target velocity and system operating frequency unless range and/or velocity ambiguities are tolerated. Narrow linear bandwidth can be obtained by either comb filter techniques which involve the complexity of a large bank of filters, or a system where a single filter is swept through the possible first order doppler spectrum at the expense of time.